There are three main types of golf balls produced by the golfing industry, the “one piece”, the “two piece” and the “three piece” ball. The “three piece” ball as the name suggests consists of three basic components: (1) a solid or liquid center section; (2) rubber windings or solid material around the center; (3) and a cover. The “two piece” ball is a newer method for producing a golf ball consisting of a solid core surrounded by a cover. Finally the “one piece” ball is a solid homogenous ball consisting solely of one material. The cover on the “two piece” and “three piece” balls are an important factor in the final properties of the ball.
Until the late 1960's compounds based on natural or synthetic transpolyisoprene covered most golf balls. Additionally, due to the relative softness of the balata cover, skilled golfers are able to impart various spins on the ball in order to control the ball's flight path (e.g. “fade” or “draw”) and check characteristics upon landing on a green. Balata-like covers are used predominantly today because of their desirable playing characteristics.
However golf ball manufacturers have been working to replace balata covers for many years because of the many problems associated with the material. While golfers were satisfied with the performance and feel of the balata covers the material is much less durable than the newer synthetic resins. Balata resins are also costly to purchase relative to other cover materials and the labor intensive manufacturing process makes golf balls produced with a balata cover significantly more expensive with a much shorter lifespan than the newer synthetic resins.
The golfing industry addressed the problems associated with ball durability and the costly manufacturing of balata covers by moving to synthetic resins. The vast majority of modern golf balls are covered with a thermoplastic resin such as “Surlyn®”, produced by E.I. Dupont De Nemours & Company (see U.S. Pat. No. 4,884,814) or “Escor®” and “lotek®” produced by Exxon Corporation (see U.S. Pat. No. 4,911,451). These resins are ionomeric polymers that undergo ionic bonding between their polymer chains to produce a material that is very durable.
Ionomeric resins are polymers containing interchain ionic bonding. As is well known in the chemical arts, ionomeric resins are copolymers made from an olefin containing two to eight carbon atoms, typically ethylene and the metal salt of an unsaturated carboxylic acid. Typical unsaturated carboxylic acids are acrylic acid, methacrylic acid or maelic acid. The copolymer then has a certain percentage of its acidic groups neutralized by a metal ion such as magnesium, sodium, zinc or lithium to produce a synthetic cover with the durability desired. The pendent ionic groups in the ionomeric resins interact to form ion-rich aggregates contained in a non-polar polymer matrix. The shortfall of these durable synthetic covers is that the materials can be very hard and the playability of the ball is decreased. The “hardness” of a golf ball can affect the “feel” of a ball and the sound or “click” produced at contact. “Feel” is determined as the deformation (i.e. compression) of the ball under various load conditions applied across the ball's diameter. Generally, the lower the compression value, the softer the “feel.”
To address the problem of hardness the golfing industry then attempted to soften the synthetic covers by blending hard ionomeric covers with the desired durability with those of a softer polymer such as a polyurethane. Unfortunately the blending of the hard ionomeric polymers with a softer polymer was difficult to process and the resulting ball cover proved to have inadequate distance capability and reduced durability.
Other attempts at blending the hard ionmeric polymer and a softer polymer were addressed in U.S. Pat. No. 4,884,814. The '814 patent blended a “hard” methacrylic acid based ionomeric resin with a softer methacrylic acid based ionomeric resin. The hard resin measured 60 to 66 and the soft resin measured 25 to 40 on the Shore D scale when measured in accordance with ASTM method D-2240. While processability of this blend was significantly better it also suffered from decreased distance.
In an attempt to overcome the negative factors of the hard ionomer covers, DuPont introduced low modulus SURLYN ionomers in the early 1980's. These SURLYN ionomers have a flexural modulus of from about 3000 to about 7000 PSI and hardness of from 25 to about 40 as measured on the Shore D scale—ASTM 2240. The low modulus ionomers are terpolymers, typically of ethylene, methacrylic acid and nB or iso-butylacrylate, neutralized with sodium, zinc, magnesium or lithium cations. E.I. DuPont De Nemours & Company has disclosed that the low modulus ionomers can be blended with other grades of previously commercialized ionomers of high flexural modulus from about 30,000 to 55,000 PSI to produce balata-like properties. However, soft blends, typically 52 Shore D and lower (balata-like hardness), are still prone to cut and shear (abrasion) damage.
The low modulus ionomers when used without high flexural modulus blends produce covers with very similar physical properties to those of balata, including poor cut and shear resistance. Worse, wound balls with these covers tend to go “out-of-round” quicker than wound balls with balata covers. Blending with hard SURLYN ionomers was found to improve these properties.
The primary properties associated with golf ball performance are resilience and hardness. The coefficient of restitution (C.O.R.), which is defined as the ratio of the relative velocity of the sphere before and after direct impact is used to determine the resilience of a golf ball. The C.O.R. is measured on a scale of zero to one, with one being a perfectly elastic collision and zero an inelastic collision.
The C.O.R. of a golf ball is a function of the properties of its core and cover combination. The golf ball with a higher measured C.O.R. performs better than other golf balls with a lower measured value. Materials with a C.O.R. of 0.700 and above are useful as a cover material. Materials with a C.O.R. of 0.715 at initial velocity of its flight are considered to have superior performance as a golf cover.